Transformer-isolated power conversion circuits can be divided into two very general categories: single ended or asymmetrical converters, and symmetrical converters. In switch mode power converters, for example, well know asymmetrical converters include flyback converters, single switch forward converters, and asymmetric half bridge or dual switch converters. Symmetric converters include push-pull converters, half bridge with capacitors (or series push-pull) converters, and full bridge converters. Single ended converters generally have fewer components, are therefore more economical, and are favored over symmetric for low power applications. However, symmetrical converter circuits have a number of features which become particularly attractive at high power levels. These features include balanced use of the output rectifiers and reduced output filter requirements. Although the details of operation may differ for such varieties as square wave or resonant transition full bridge circuits, all well-known bridge or symmetrical drive circuits apply an alternating positive and negative voltage to the primary winding of the power transformer. In the transition period between the application of positive and negative voltage, there is an intervening zero voltage or freewheel time interval as may be required to accomplish regulation.
For high output voltage levels, such as might be used for motor drives or relays, the output from the secondary winding of the transformer is typically rectified using four rectifiers in a full bridge rectifier arrangement. However, for applications requiring low voltages and high currents, such as high performance logic circuits, the two rectifier forward voltage drops encountered in a full bridge rectifier will cause an unacceptable power loss. For these low voltage, high current applications, a center tapped secondary winding is typically used in conjunction with two rectifiers to obtain full wave rectification. This arrangement eliminates the unacceptable power loss due to the full bridge configuration since only one rectifier forward voltage drop is encountered.
In very high current applications, conventional wire cannot be employed as conductors in the secondary structure precisely because of the high current passing through the conductor. Typically, bus bars or copper plates or disks are used to accommodate the high current in these types of transformers. A significant difficulty in using either a bus bar or a copper plate or disk for the secondary structure is that it is mechanically awkward to make the three required connections (two rectifier and one center tap), which occur at one end of the transformer. FIG. 1 illustrates an exploded view of a conventional prior art bridge transformer and a typical manner of making the connections to the secondary structure. Elements 10 and 20 are the familiar C--C magnetic core halves, which when mated form a continuous magnetic flux path. Encircling post 25 of the core are the primary winding, 30 and 40, and the secondary winding, 50 and 60. As illustrated in FIG. 1, the secondary structure is constructed from three distinct elements, the two copper disks, 50 and 60, and the center tap piece 70. The center tap piece 70 in the secondary structure performs a traditional center tap function, but also serves as a level jump to connect the other two levels of the secondary structure, 50 and 60. The anode sides of the two rectifying diodes, 80 and 90, are shown connected to the secondary disks, 50 and 60, respectively, while the cathode sides of the rectifiers are shown commonly connected to the external circuit (shown as a filter-load comprising a LC filter and a load resistor R in FIG. 1). The other connection to the external circuit is shown from the center tap 70. As can be seen from FIG. 1, the prior art transformer structure requires that all three connections (secondary disk 50 to rectifier 80; secondary disk 60 to rectifier 90; and center tap 70 to the filter-load circuit) be made on the same side of the transformer, namely at or near core post 25. The mechanical difficulty of accomplishing these connections manifests itself most profoundly in the fabrication, assembly and mounting of the transformer.
Apart from the fabrication, assembly and mounting problems, the mechanical congestion of the connections can have a direct, degrading effect on electrical performance. The connection congestion interferes with the short, low inductance rectifier connections necessary for operation at high switching frequencies. Both the rectifier connections to the secondary and the rectifier connections to each other should be as short as possible to maintain low inductance. Having both rectifier connections and the center tap connection on the same physical side of the transformer increases the difficulty and therefore the length of these connections.
In high current transformer applications, the transformer will typically provide a large voltage step-down ratio and the different structural composition of the primary and secondary windings required to achieve this step-down will provide additional fabrication and assembly challenges. In such a transformer, the primary winding will contain many turns (typically on the order of 15 to 50) of relatively flexible conductor, such as wire, flat wire, or braid. In FIG. 1, the primary winding is shown as divided into two substantially symmetrical halves, 30 and 40, wired in a series aiding fashion. Each primary winding half is a planar, multi-turn coil of wire conductor. The primary will carry modest currents of 5 to 20 amps, peak alternating current (AC) with no direct current (DC) component. The primary winding must be insulated to withstand hundreds of volts to function, and perhaps thousands of volts to meet safety requirements. In comparison to the primary, the secondary structure contains relatively massive and inflexible parts, such as the copper disks 50 and 60 shown in FIG. 1. The secondary structure will carry hundreds of amps with both AC and DC components. Fifty volts functional insulation is adequate for the secondary.
The vastly different characters of the primary and secondary structures of such a transformer are likely to require different fabrication, assembly, and mounting techniques. Yet, in order to provide tight magnetic coupling between the primary and secondary structures, which is required by any high performance transformer, it is necessary that the two structures be in close physical proximity. To further improve the transformer coupling, the primary and secondary windings are often arranged in complex interleaved patterns or in a sandwich pattern, such as the one illustrated in FIG. 1. In that figure, the windings are all mounted on core post 25 with the secondary disks 50 and 60 being sandwiched between the primary coils 30 and 40.
While such arrangements improve the transformer performance, they introduce fabrication and assembly difficulties by requiring the intermingling of structures of vastly different character. Further, these sandwich or interleaved arrangements complicate thermal management in high power converters, since heat must flow across several interfaces between windings, in order to reach a cold plate or heat sink.
It is therefore one object of this invention to provide the electrical function of a conventional bridge transformer without the associated connection congestion normally associated with such a transformer.
It is also an object of this invention to provide tight magnetic coupling without the need for sandwiching or interleaving the windings of the transformer.
It is another object of this invention to facilitate fabrication assembly and mounting of a transformer structure.
It is an additional object of this invention to improve electrical circuit performance such as increased switching frequency.